1. Field of the Invention
The present invention relates to a pressure controlling device for controlling pressure acting on bearings provided in a Roots blower type supercharger mounted on an internal combustion engine.
2. Description of the Related Art
A Roots blower type supercharger of an internal combustion engine has two mating lobed impellers fixed on two rotary shafts, respectively, so that these impellers rotate in opposite directions to each other to carry out a pumping operation. There are usually clearances between the two impellers and between the impellers and an inner surface of a housing in which the impellers are housed, and accordingly, a part of the highly pressurized air discharged by the impellers may leak to a low pressure part in the housing through these clearances. As a result, pressure of the leaked air acts on one end face of bearings which rotatably support the rotary shafts, so that a pressure difference occurs at the end faces of each bearing. This high pressure causes grease lubricating the bearings to flow to the low pressure side of the bearing. In other words, the grease holding ability of the bearings is reduced. To prevent such a reduction of the grease holding ability caused by this pressure difference, a construction by which air pressure acting on the bearings is reduced has been proposed. That is, this construciton comprises a labyrinth formed on an inner surface of a hole through which the shaft extends, and a portion between the bearing and the labyrinth is communicated with the atmosphere, so that the above pressure difference is reduced.
In the above construction, a clearance between the labyrinth and the shaft must be rather large, to prevent contact between the labyrinth and the shaft, and an opening for connecting the portion between the bearing and the labyrinth to the atmosphere also must be large to fully reduce the air pressure acting on the bearings. With a construction having such a large clearance and opening, however, in an engine state in which the amount of intake air is considerably reduced, such as idle running, a problem occurs in that the atmospheric air is sucked into the intake passage of the engine through the labyrinth so that the amount of intake air is increased, that is, the number of revolutions of the engine is increased even though the throttle valve is closed.
To resolve this problem, in Japanese Unexamined Patent Publication No. 61-16232, the applicant has already proposed a construction in which an annular space is formed around the shaft and between the bearing and the labyrinth, and this space can be communicated with the atmosphere. The annular space is communicated with the atmosphere when supercharging occurs, and is shut off from the atmosphere when the amount of intake air is reduced to a minimum.
In the above proposed device, however, since the annular space is communicated with the atmosphere, during a sudden increase in the engine load a part of the boost pressure generated in the supercharger is released to the atmosphere through the annular space. As a result, the desired output torque of the engine is not fully realized during this sudden increase in engine load, so that the resultant acceleration of the vehicle is poor.